Microwave devices are electronic devices that operate in the frequency range of approximately 300 MHz to 300 GHz. Passive microwave devices include resonators, filters, and delay lines. Active microwave devices include amplifiers, receivers, and signal processing circuits. Microwave components can be made with normal metals or with superconductors. While normal metal technology is more mature, superconductive components often exhibit advantages in size, speed, and signal fidelity.
Devices, circuits, and systems operating at microwave frequencies require packages whose own frequency response does not limit or interfere with the performance of the components. Furthermore, even microwave devices made from normal metals perform better when cooled to low temperatures. Any package for these devices must therefore maintain its electrical and physical integrity over a wide temperature range.
Delay lines are used for short term storage of information in microwave circuits and systems. It is often desirable to make the delay time as long as possible without introducing excessive loss, or degradation of signal quality. Loss or degradation of the signal is controlled by both the inherent properties of the delay line and substrate materials, and the physical arrangement of the delay line device. Materials properties that are desirable for a delay line are low surface resistance in the delay line material and a low dielectric loss in the substrate material. Device designs or patterns should be chosen to decrease dispersion. These requirements sometimes compete, necessitating a compromise in the final device structure.
For a given configuration, the delay length, and therefore, the amount of short term storage available, may be increased in several ways. The line itself may be lengthened, narrowed, or patterned onto a thinner substrate. A longer line requires either a larger substrate area or a smaller spacing between parts of the line for tighter packing. A narrower line requires the width of the feature to be reduced. The substrate thickness is obviously reduced in the third option, leading to greater fragility. All of these options lead to increased complexity of processing, either because of the area over which a pattern must be maintained uniformly, the minimum feature size that must be repreoduced, or the care which must be taken to ensure the unity of the ultimate product.
It would be advantageous to process parts of the delay line structure independently and later put the pieces together to form a long delay line. This modular approach would allow pieces to be fabricated using comfortable technology, while providing an end product which fulfills the ultimate goal of a long delay line. Because there would now be many pieces to hold together, however, the modularity of the design would lead to more complex packaging requirements for the ensemble.